For centuries individuals have lifted heavy objects in order to increase their strength and fitness. More recently, the advent of compact weight lifting machines has enabled users to perform multiple lifting exercises on the same device. A common feature of these machines is that they enable the user to select from multiple available weights or resistances, thus allowing the user to adjust the amount of effort necessary to perform any given lifting exercise. U.S. Pat. No. 5,135,451 to Chen discloses one such lifting machine in which the resistance is provided by a stack of weights located at the back of the device, and is incorporated by reference herein in its entirety.
Although these machines provide a convenient and space-efficient means for performing multiple lifting exercises, the arrangement of these machines makes it difficult to perform certain types of lifting workouts. If a user wishes to cycle through multiple resistance levels in the course of a single set, the user must either enlist the aid of a partner to adjust the weight, or stop the workout, adjust the weight, and then resume the workout. Thus, a lifting machine which allows a user to adjust the effort necessary to perform the workout mid-set without stopping the workout and without the aid of another person is desired.
U.S. Pat. No. 5,788,616 to Polidi discloses a weight lifting machine that utilizes a user-controlled electric motor to adjust the resistance experienced by the user during the workout, and is incorporated by reference herein in its entirety. One drawback of such a machine is that it requires access to an electrical power source to operate. Additionally, electrical components can be difficult and costly to replace in the event they break down. Thus, there is a need for a user-controlled, resistance-adjusting machine that does not rely on electrical components. Furthermore, the expense associated with replacing an existing weight machine can be cost-prohibitive for both individual users as well as commercial fitness centers. Accordingly, it is desirable that any user-controlled weight adjustment device be able to attach to existing weight lifting machines.